JP-A-1-263997 discloses a technique for make more strict judgment criterion for verify in comparison to those in the cases of typical read operations for a nonvolatile semiconductor memory, in which complementary data are written in a pair of memory cells, a potential difference between paired bit lines produced by data read out from the pair of memory cells is amplified by a differential sense amplifier, and then the resultant readout data is judged. In addition, JP-A-2004-319007 discloses a multivalued storage technique by which data of quaternary or more are held in a pair cell constituted by a pair of nonvolatile memory cells.